1. Field of the Invention
The invention relates to a switching converter and a method for regulating a clocked buck/boost converter, wherein a buck converter switching element is driven at a common clock frequency with a first pulse-width-modulated switching signal and a boost converter switching element is driven with a second pulse-width-modulated switching signal to convert an input voltage into a regulated output voltage, and wherein a regulator signal from an output voltage regulator is used to generate the pulse-width-modulated switching signals.
2. Description of the Related Art
Buck/boost converters have long been known. These devices essentially involve switching converters that operate alternately or in a transitional area simultaneously in the manner of a boost converter and in the manner of a buck converter. In a corresponding circuit arrangement, a common choke as well as a common output filter are provided. As a rule, the buck converter switching element is connected between the input voltage and the first terminal of the choke. The reference potential of the input voltage is connected, on one hand, to the reference potential of the output voltage and, on the other hand, via a first diode likewise to the first terminal of the choke. This second terminal of the choke is additionally connected via a second diode to the terminal of the output voltage (see FIG. 1). In this case, the diodes can also be embodied as synchronous switches.
If the input voltage exceeds the output voltage, the switching converter operates as a buck converter. As soon as the input voltage drops below the output voltage, conversion switches to the boost converter mode (see FIG. 2).
As a rule, the two switching elements are driven by pulse-width-modulated switching signals. To form these switching signals, a regulator signal of an output voltage regulator is mostly overlaid onto a sawtooth or triangular signal. An exemplary buck/boost converter is disclosed in patent application DE 43 06 070 C1, where two pulse-width-modulated signals are generated with only one regulator signal. In this case, one sawtooth signal is displaced in relation to the second sawtooth signal by a value equal to the amplitude. This means that with an increasing regulator output signal, the duty cycle of the first pulse-width-modulated switching signal for driving the buck converter amounts to 100% before the second switching signal delivers first switching impulses to the boost converter.
To compensate for voltage drops in the components of the switching converter, methods are also known in which the two sawtooth signals are displaced by a value equal to the amplitude minus a correction value. Such overlaying of the sawtooth signals for realizing an overlapping operation of the boost and buck converter is known from U.S. Pat. No. 6,166,527 A. It is known from WO 2009/033924 A2 that, during a transition from buck converter operation to boost converter operation, the boost converter switching element can be synchronized with an earlier clocking. In this case, a further pulse-width-modulated switching signal is generated that specifies an earlier clocking on transition.
A fault-free transition between the two operating modes is, as a rule, only possible if the switching elements of the buck/boost converter are connected in a continuous mode. The common choke in this case always remains magnetically charged and no resonance oscillations occur, which conventionally occur after the magnetization of the choke has taken place. The respective switching element is switched into a discontinuous mode after the choke has been de-magnetized. A dead time thus occurs. According to the prior art, the aim in a discontinuous mode is quasi-resonant switching (i.e., valley switching). In this case, the switching element is switched on when the voltage oscillating at a resonant frequency at the switched off switching element is at a minimum. In this way, turn-on losses are kept low.
With a generic buck/boost converter, unspecified resonance events within existing resonant circuits can lead to faults. Such faults involve increased losses, for example, because no valley switching can occur. In addition, faults occurring can lead to undesired noise emissions.